Sensory hair cells are essential for the transduction of mechanical stimuli into hearing and balance signals in the inner ear. Damage to these hair cells is the major cause of sensorineural hearing impairment, which affects more than 17 million Americans. Recent studies have demonstrated in mammals the capability of damaged hair cells to regenerate in vitro. These observations provide a rationale for the treatment of hearing impairment by regenerating hair cells. This treatment is plausible since several neurotrophic molecules (i.e. TGF-alpha, retinoic acid, and bFGF) have been implicated in the regenerative process. Furthermore, BDNF and NT-3 are believed to maintain the survival of spiral ganglion neurons (which innervate hair cells), and may also be able to aid in hearing restoration. As a first step towards a possible therapy for hearing impairment, this project proposes to use viral vectors to deliver a reporter gene to the inner ear of guinea pigs, and to assess the efficiency of the delivery, the toxicity of the vectors, and the expression pattern of the marker gene within different cell types. Four replication-defective viral vectors (HSV, adenovirus, AAV, and retrovirus), all bearing the marker gene lac Z will be injected into the round window of the inner ear. High titer vectors will be used since the total volume of the cochlear fluid in the guinea pig is small (approx. 3-4 microliters). After injection, the animal will be sacrificed at 2 days or l week, and the expression of the lac Z gene within the cochlea will be evaluated histochemically. A second goal of the project is to assess the capability of the four viral vectors to deliver neurotrophic molecules to the inner ear in the hope that they may be useful in repairing hair cells or spiral ganglion neutrons that have been damaged by ototoxic drugs. Different vectors will contain cDNA constructs for BDNF, NT-3, and bFGF. After injection, the animals will be sacrificed at 2 days or l week, and surviving hair cell and spiral ganglion neurons will be counted. These models have clinical application, as they will lay the groundwork for the development of a gene therapy for hearing loss.